FIG. 14 shows a conventional example of a gear six-speed transmission for a motorcycle. An input shaft 201 for shifting and an output shaft 202 for shifting are provided in parallel with each other in a transmission case (not shown). The input shaft 201 has a speed change input gear group. The speed change input gear group includes first-speed to sixth-speed input gears 211, 212, 213, 214, 215 and 216. The input shaft 201 is coupled to a crankshaft (not shown) via a clutch 219 so as to be capable of transmitting power. The output shaft 202 for shifting has a speed change output gear group. The speed change output gear group includes output gears 221, 222, 223, 224, 225 and 226 which engage with the input gears 211, 212, 213, 214, 215, and 216 at all times.
In the conventional example, the first-speed input and output gears 211 and 221, the fourth-speed input and output gears 214 and 224, the third-speed input and output gears 213 and 223, the fifth-speed input and output gears 215 and 225, the sixth-speed input and output gears 216 and 226, and the second-speed input and output gears 212 and 222 are arranged in this order from the clutch 219 side.
A shift mechanism will be described. Regarding the input shaft 5, the third-speed input gear 213 and the fifth-speed input gear 215 are arranged at the center of the input shaft 201 in the axial direction. The third-speed input gear 213 and the fifth-speed input gear 215 are integrally formed on each other and spline-fitted on the input shaft 201 so as to be movable in the axial direction. The remaining first-speed, fourth-speed, sixth-speed and second-speed input gears 211, 214, 216, and 212 are fixed to the input shaft 201 so as not to be movable in the axial direction. Then, a dog teeth clutch mechanism 231 is provided between the fourth-speed input gear 214 and the third-speed input gear 213, and a dog teeth clutch mechanism 232 is provided between the fifth-speed input gear 215 and the sixth-speed input gear 216.
On the other hand, regarding the output shaft 202, the fourth-speed output gear 224 and the sixth-speed output gear 226 are spline-fitted on the output shaft 202 for shifting so as to be independently movable in the axial direction. The remaining first-speed, third-speed, fifth-speed and second-speed output gears 221, 223, 225, and 222 are fixed to the output shaft 202 so as not to be movable in the axial direction. Then, a dog teeth clutch mechanism 233 is provided between the fourth-speed output gear 224 and the first-speed output gear 221, and a dog teeth clutch mechanism 234 is provided between the sixth-speed output gear 226 and the second-speed output gear 222.
Although not shown, the shift operation mechanism including a shift cam drum and shift forks movably operates the third-speed input gear 213 and the fifth-speed input gear 215, the fourth-speed output gear 224, and the sixth-speed output gear 226 in the axial direction. This moving operation selectively engages the dog teeth clutch mechanisms 231, 232, 233 and 234 with the gears, thereby shifting to the desired shifting stage. The above configuration is disclosed in Patent Document.